So I tried to make a simple tensioning system to use with the kit chassis. Basically, 1" square aluminum extrusion fits inside the chassis rail, and slots are milled into the chassis rail so that the piece of extrusion can slide back and forth. With one of these on each side, wheels can slide back and forth slightly to get perfect chain tension. Has anyone done something similar to this? How well did it work?

I do see one potential problem with it…since it’s a square tube, and there is nothing surrounding the bolt, the tube will crush before you get the bolts tight enough to securely clamp the tube in place so it won’t be slid by chain tension under load.

You could also mill the blocks from either solid aluminum or maybe even delrin to avoid the crushing problem. That is an awesome Idea and I think there would be a pretty awesome market for such a thing.

I do see one potential problem with it…since it’s a square tube, and there is nothing surrounding the bolt, the tube will crush before you get the bolts tight enough to securely clamp the tube in place so it won’t be slid by chain tension under load.

we solved that problem by adding a piece of PVC pipe as our space for a similar application. It worked quite well.

That might work a bit better if you had two shorter slots, one for each of the bolts, and you limited the play as much as possible. Use it to take up the slack in a chain or belt, but don’t use it to reposition the wheel significantly. (So you’d only need, at most, one chain or belt pitch worth of play.)

Also, I assume this is intended principally for a dead axle system. If not, you’re going to have a hard time rotating the shaft, if there’s any misalignment.

And while you’re at it, you probably want to use some flat washers under the bolt heads and nuts, so that the tightening process doesn’t embed the hexagonal surfaces into the chassis rails. This assumes that you’re going to use nylon locknuts and will design the system so that you can apply significant bolt tension without crushing the channel.

is that really a problem because of the fact that all the tightening is happening on the c channel and not the square channel

Yes, it is really a problem. Bolts work best when they are clamping something solidly between them, so they can squeeze the part very tightly. You could try it and see what happens, experiments are a good way to learn the effects of certain design principles.

Adding in a lateral screw adjustment component can help avoid teh need for clamping forces. Look at the rear axle of most chain driven motorcycles. See if you can come up with something similar to that, and then clamp load is nearly irrelevant.

That is an awesome Idea and I think there would be a pretty awesome market for such a thing.

I hadn’t considered marketing anything, but your comment made me think, if it were to be marketed, teams wouldn’t want to mill out their kit chassis. Then I realized, the slots could be made in the piece of extruded aluminum, and not in the chassis itself. It seems to me that this would be a nicer way than my original design because a team could try it without doing anything to their chassis, and take it out later if they wanted. I’ll try to design this other way in CAD as soon as I get the chance and a post picture.

Also thanks for the comments. Washers should definitely be there, and tubing around the bolts is also a very good idea.

I used a similar method in 2003. But be careful
the slots are in the same direction that the pull of the chains will be…
So when the motor pulls it could slide the screw to the loose side. It will be difficult to tighten the screws enough to keep this secure. You might add a second screw (and spacer) to each side directly in the slot.

If you really jam your motors on… (in quick acceleration or in changing from one direction to the other…) you can put a great deal of force in the same direction as the slots…if one side is less tight than the other you will get misalignment and you will throw chains…

You might want to consider something that moves the motor in and out perpendicular to the pull of the chain… this would not have as great a tendancy to loosen. There are many ways to keep tension on chains…
I am sure if you do a search you will find others.

Testing is the key… keep track of how tight you tighten the bolts… (use a torque wrench) Don’t count on tightening as tight as you can without being able to know how tight that really is… (this can vary between people and between situations…) Reproducibility and testing the system under these shock loads will be necessary for you to see if this will work for your situation.

Just be wary of the nature of this design… As I said… back when we were using drill motors and a plastic transmissions this method caused us a great deal of grief…and matches with our robot traveling in a circle on the field while dangling a chain…

Keep it simple. A solid wooden block which fits snugly inside the square tubing will keep it from crushing. You can make it a mechanical tensioner by sliding an eye bolt over one of the lock down bolts and running through a fixed point.

If you were truly looking to market and sell the tensioner I would push for it to be milled out of aluminum. This would eliminate having multiple pieces of tube or what not to avoid crushing the square tube, and if you thread one end for say 1/4-20 then whoever bought the tensioner could simply use the appropriate sized 1/4-20 bolt as a tensioning device.

Marketing these was certainly not my initial intention in designing them, which is why they are made from aluminum tube and why they do not require much machining. My team also doesn’t have the capability of producing any large number of these. However, if they do work, I imagine they could be quite useful. I plan to make a few variations of these in the next week or so to test them out. If they do prove to be successful, AndyMark would be the company to market them if they were interested.

When you try out this idea, I suggest you look up the torque specification for the size of bolt you plan to use. Get a torque wrench, and see what happens to the tube and channel when you tighten the bolt to spec torque with a spacer, then remove the spacer and try again and see what happens. I’d be interested in the results.

We did something similar but used a short section of 1" 80-20 instead of tube. No need to slot the frame, the slot in the 80-20 worked fine. We just drilled a 3/8" hole through the 80-20 to hold the axle. We used 2 bolts from the bottom up into the slots. I think this is about the same thing you proposed in your second post. It worked really well but I’ll be interested to see if it will hold up when the floor isn’t so slippery. The loads on the wheels were pretty low this year. I guess a scheme with a bolt threaded into the end of the 80-20 would make it stronger as would using more bolts from the top or side.

One thing that was a problem was that a couple of our corner blocks were a little thin. The channel got a bit squished and the wheel mounts were too tight. We solved this on the belt sander but it was kind of crude. Next time we’ll shim around the corners if needed.

You have a good design there, lighter than the 80-20. I’m curious if your axle holes got oblong. I was a little worried about ours but it seemed to hold up fine for the one regional we went to.

Yea the chasis will flex when you tighten the bolt… thats what happend to our robot this year. To fix it all we did was just cut aluminum blocks, drill a hole for the bolt to fit in, slide it into the channel of the chasis, and then tighten.